1. Field of the Invention
This application relates broadly to plant food particles and to methods and compositions for their production. More particularly, it concerns controlled release, particulate fertilizer products comprising a water soluble fertilizer core encased in a plurality of water insoluble, abrasion resistant coatings and unique coating compositions and methods for the production of such products.
2. Description of the Prior Art
Commercial particulate fertilizers are produced and marketed in several different particle types, i.e., granular, pelletized and prilled fertilizers. Also they may be formed of inorganic substances, organic substances, or combinations thereof. The improvements of this invention can be applied to any of these.
To be effective in promoting plant growth, fertilizers must contain some amount of water soluble plant nutrients, typically in the form of water soluble compounds of nitrogen, phosphorus and potassium, alone or in combination, and often in conjunction with other elements, e.g., calcium, boron, magnesium, zinc, chlorine, etc. Such particulate fertilizers can be made of a single component, e.g., urea, ammonium nitrate, potassium chloride, etc., or of multiple components often mixed with inert water soluble or water insoluble materials as in common fertilizers designated 6-6-6, 4-6-4, 10-10-10, 20-20-5, 14-16-0, 5-20-20 and the like. In addition, specialized fertilizers may contain optional additives such as herbicides, insecticides, trace elements, iron salts, etc. The improvements of this invention can be applied to any of these.
Historically, particulate fertilizers possessed a number of known defects, the most notable being the rapid release of soluble plant food, causing phytotoxicity and the rapid depletion of the nutrients by leaching. Other problems included tendencies to cake and form dust. These problems are well documented in prior art patents that professed solutions to one or more of the know defects including U.S. Pat. Nos. 3,475,154; 3,259,482; 3,263,088; 3,264,089; 4,711,659 & 4,772,490 and Japanese patent 52-38361. The present invention provides further improvements in the construction of particulate fertilizers that render them highly attrition resistant and noteworthy in extended release properties.
A number of slow release coatings for particulate fertilizers have been proposed. The process of sulphur coating urea particles is described in U.S. Pat. No. 3,342,577 and was developed in 1968 by the Tennessee Valley Authority (TVA) as an economical system for reducing the rate of dissolution when urea particles are applied to the soil as fertilizer. This process requires high levels of sulfur, which reduces the nutrient analysis of the fertilizer particles, and even then imperfections remain in the coating making it necessary to apply a sealant coating composed of a mixture of 30% polyethylene resin in 70% bright stock mineral oil.
Coatings in which preformed polymer resins are applied from solvents have been proposed in, for example, U.S. Pat. Nos. 3,475,154 and 3,264,089. The use of solvents creates a vapor hazard as the products are dried and the solvent evaporation step can cause pinhole imperfections in the coatings when applied.
Polyurethane coatings as proposed in U.S. Pat. Nos. 4,711,659 and 4,969,947 require that the substrate contain a minimum quantity of reactive --NH.sub.2 groups and thus are not applicable to all fertilizer compositions for which slow release properties are desirable.
An additional problem encountered with coated urea products is the reduction or loss in water insoluble nitrogen (WIN) which occurs due to abrasion encountered in modern high speed blending and bagging operations.
Urea and blended fertilizers when they are mixed and bagged must be labeled with their WIN content, and it is the responsibility of the manufacturer to assure that the value does not decrease below the labeled value during shipping and storage. Thus, a significant loss in WIN resulting from handling in modern high speed equipment can create a situation in which the fertilizer is mislabeled and subject to recall. The present invention makes it possible for fertilizer manufacturers to produce bagged particulate fertilizers that maintain their WIN content during shipping and storage.